I say 2 months is short but I think 3 months could be ok. 3 and a half months would be better even for holidays.

My plan to 66 is get 5k xp a day. That would take 20 days but there are holidays. But there could be the players who are super busy. They still want 66 but only average about 1k xp a day. Since 3 months is about 90 days I say there should be 3 and a half months break between next cap. Why and a half? Because there are many holidays like Thanksgiving and Christmas. People will probably go on a 2 day-2 week vacation for christmas and might not have time to play. Instead they probably will celebrate with their families and have fun on their vacation.